The present invention relates to an apparatus for feeding slurry to a chemical mechanical polishing (CMP) apparatus during a semiconductor fabrication process and a method for preparing slurry.
During a semiconductor fabrication process, a CMP device polishes a film, which is applied to a wafer surface and formed from metal such as tungsten or copper, with a chemical solution that includes a polishing agent. The chemical solution is a slurry that is prepared by mixing a polishing agent and an oxidizing agent in a stock solution. To fabricate semiconductor devices with uniform circuit wiring dimensions and increase yield, the concentration of the oxidizing agent in the slurry must be maintained at a constant value.
In the prior art, the polishing agent is formed from abrasive grains, such as silica, alumina, or cerium, and the oxidizing agent is formed from ferric nitrate. The pH of a mixture of the polishing agent and the stock solution (slurry stock solution) differs greatly from the pH of the oxidizing agent. The mixing ratio of the slurry stock solution and the oxidizing agent (slurry stock solution: oxidizing agent) is 1:1 or 2:1. The concentration of the oxidizing agent in the slurry may be obtained by measuring the pH after the slurry stock solution and the oxidizing agent is mixed.
However, chemical reaction between the polishing agent and the oxidizing agent tends to coagulate the abrasive grains. The abrasive grains precipitates within a short period especially when alumina is used as the abrasive grains. This results in an instable polishing rate and scratches the polishing surface with the coagulated abrasive grains. Therefore, aqueous hydrogen peroxide (H2O2) is nowadays used as the oxidizing agent.
The pH of the aqueous hydrogen peroxide is about 7.0 and neutral, and the mixing ratio of the slurry stock solution and the oxidizing agent is 10:1 or greater. Thus, the pH of the mixture does not change much when the oxidizing agent is added to the slurry stock solution. As a result, the concentration of the oxidizing agent cannot be obtained from the pH.
To measure the concentration of the aqueous hydrogen peroxide in the slurry, the incorporation of an automatic titration device in a chemical solution feeding apparatus has been proposed. However, titration analysis requires at least about ten minutes to perform a single analysis. Therefore, the concentration of the mixture cannot be constantly monitored even when using the automatic titration device.
Further, a reagent is used to conduct the titration analysis. The reagent must be replenished when it becomes insufficient. The adding interval becomes shorter when the titration analysis interval is shortened. This causes the replenishment of the reagent to be burdensome. Further, a drainage process must be performed to purify the waste liquid produced by the titration analysis.
The aqueous hydrogen peroxide dissolves in the slurry. Thus, as shown in FIG. 9, the concentration C of the aqueous hydrogen peroxide in the slurry decreases as time elapses. To maintain the concentration of the oxidizing agent at a constant value, the concentration of the aqueous hydrogen peroxide must be measured in order to replenish the aqueous hydrogen peroxide when it becomes insufficient.
The automatic titration analysis is optimal for performing concentration detection to replenish insufficient aqueous hydrogen peroxide. However, when detection results must be obtained immediately to constantly check the concentration of the oxidizing agent, the automatic titration apparatus should not be employed.
The concentration of the stock solution of the aqueous hydrogen peroxide is not constant since the aqueous hydrogen peroxide vaporizes. Accordingly, even if the slurry stock solution and the aqueous hydrogen peroxide are mixed at a predetermined mixing ratio, the concentration of the aqueous hydrogen peroxide in the slurry does not remain constant and may thus exceed a predetermined concentration.
In this case, the slurry stock solution must be replenished again. Then, the aqueous hydrogen peroxide must be replenished, and the concentration of the aqueous hydrogen peroxide must be adjusted. This is burdensome.
Further, after preparing slurry having a predetermined concentration, the aqueous hydrogen peroxide reacts with the slurry as time elapses and deteriorates the slurry components. This fluctuates the polishing rate.
Japanese Laid-Open Patent Publication No. 11-126764 describes a slurry feeding apparatus having two tanks to constantly feed fresh slurry to a polishing machine. In each tank of the double-tank slurry feeding apparatus, the preparation of the slurry and the feeding of the prepared slurry until the slurry is emptied are performed alternately. Accordingly, unless the concentration of the aqueous hydrogen peroxide is accurately adjusted when the slurry is prepared, the concentration of the aqueous hydrogen peroxide may differ between batches.